This invention relates to a digital signal processing device capable of calculating real and imaginary parts of an input signal and evaluating the phase and envelope of the input signal from the real and imaginary parts calculated.
In the case of an ultrasonic Doppler blood flowmeter, it is necessary that the phase of a Doppler signal be calculated so as to detect the speed and direction of flow of blood cells. The conventional phase detection method usually employs the Fourier transformation. According to the Fourier transformation method, however, data evaluation cannot begin until all the data to be used for Fourier transformation have been stored in memory. As a result, the corresponding storage time becomes a wait time and therefore increases the length of time required to perform such a transformation. In order for the ultrasonic Doppler blood flowmeter to detect and display the speed of bloodflow in real time, it is necessary for the number of data in a Doppler signal for Fourier transformation to be reduced, thereby reducing the accuracy of the readings obtained. Even if the data of a region not calculated in this way is to be evaluated by means of an interpolation method, any better accuracy is not obtained from this method and thus a time-consuming operation results. Furthermore, in typical ultrasonic diagnostic device it is necessary to detect the envelope of echo signal in order to obtain the tomographic image. The envelope of the echo signal represents the luminance of the image along a scanning line. Therefore, the tomographic image is obtained by scanning the object and by arranging the images with the detected luminance in a two-dimensional manner.
In the conventional method the envelope has been obtained by detecting the echo signal (analog signal) and passing the detected signal through a low-pass filter. However, since the waveform of the echo signal is degraded by its passage through the low-pass filter, the diagnostic data thus obtained is rendered useless due to a lowering of the spatial, densIty resolution of the ultrasonic image, as well as a lowering of the S/N ratio.